Effects seen in intensity and timing of wind-driven upwelling
SANTA CRUZ, CA--Global warming could have profound effects on the wind-driven upwelling of deep ocean water along the California coast, according to recent studies by researchers at the University of California, Santa Cruz. The studies showed changes in both the intensity and the seasonal timing of the upwelling, which brings cold, nutrient-rich water into coastal ecosystems.

This seasonal upwelling supports California's diverse marine life and productive fisheries, but how changes in the upwelling will affect these and other aspects of coastal ecosystems remains uncertain. The researchers, led by professor of Earth sciences Lisa Sloan, used computer simulations of the regional climate to show that wind-driven upwelling along the California coast will likely intensify over the next 50 years as a result of increased concentrations of carbon dioxide in the atmosphere. In addition, the models showed the upwelling season extending later into the fall.

The first set of experiments was published in the August 14 issue of Geophysical Research Letters (GRL), and a second paper with additional findings will be published online this week by the Proceedings of the National Academy of Sciences (PNAS).

Previous studies by other researchers found that the intensity of upwelling along the California coast has been increasing over the past 30 years, leading some to speculate that the trend is a result of global warming, said Mark Snyder, lead author of the GRL paper. Snyder earned his Ph.D. this month working with Sloan.

"Some people think we may already be seeing the effects of climate change on the upwelling regime, so we thought we would use our climate models to see how increases in greenhouse gases would affect the winds that drive the upwelling," he said.

These winds are the result of differences in atmospheric pressure over the land and the ocean that develop because the land surface heats up faster than the ocean surface. Similarly, global warming could be expected to raise temperatures more over land than over the ocean, making the differences in atmospheric pressure even greater and creating stronger winds. Sloan's group set out to test this hypothesis using powerful computers to run complex models of the climate system.

The researchers used a high-resolution regional climate model, centered over California and driven by inputs from a global climate model with coarser resolution, to look at the effects of rising concentrations of carbon dioxide in the atmosphere. Concentrations of carbon dioxide, a greenhouse gas that traps heat in the atmosphere, are increasing due to emissions from the burning of fossil fuels.

The first set of experiments, published in the GRL paper, showed that with increasing carbon dioxide in the atmosphere, the intensity of upwelling is decreased in the early season (April to May) and is dramatically increased during the peak season (typically July to August). The results also showed the duration of the upwelling season extends further into the fall.

"The increase we saw in the upwelling intensity in the peak season supports some of the observational work showing a trend toward increased upwelling intensity in the present day," Snyder said.

The new results just published by PNAS reinforce these findings. In these experiments, the researchers included the effects on the climate system of changes in vegetation that are likely to occur with increased atmospheric carbon dioxide. The results showed the same general effects on upwelling as in the previous studies, only more pronounced, said postdoctoral researcher Noah Diffenbaugh, lead author of the PNAS paper.

"The interactions between vegetation and climate that are set in motion by increasing carbon dioxide concentrations enhance the effects on the upwelling regime," Diffenbaugh said.

The projected changes in vegetation result from warmer and dryer conditions created by global warming and lead to changes in the overall energy balance of the land surface, he said.

Diffenbaugh cautioned, however, that despite the complexity of the climate models, they still represent a simplified view of an extraordinarily complicated system of feedbacks and interactions. Nevertheless, as human activities continue to pour more carbon dioxide into the atmosphere, these studies show just how wide-ranging its effects may be.

"The regional climate model is certainly the best tool we have at the moment, and it shows us that elevated carbon dioxide is important not only for its direct effects on the climate, but also because it induces other changes that feed back into the climate system, and the magnitudes of those feedbacks appear to be substantial," Diffenbaugh said.

Researchers at UCSC and elsewhere are currently working to understand how changes in upwelling conditions will affect fisheries and the dynamics of coastal ecosystems. The new climate studies highlight the importance of those efforts, Snyder said.